Fiber-optic communication is widely used in applications such as telecommunications, communication within large data centers, and communications between data centers. Because of attenuation losses associated with using shorter optical wavelengths, most fiber-optic data communication uses optical wavelengths of 800 nm and longer. Commonly used multimode and single mode optical fiber uses wavelengths between 800 nm and 1675 nm. A main component of optical receivers used in fiber-optic communication system is the photo detector, usually in the form of a photodiode (PD) or avalanche photodiode (APD).
High-quality low-noise APDs can be made from silicon. However, while silicon will absorb light in the visible and near infrared range, it becomes more transparent at longer optical wavelengths. Silicon PDs and APDs can be made for optical wavelengths of 800 nm and longer by increasing the thickness of the absorption “I” region of the device. However, in order to obtain adequate quantum efficiency (also known as external quantum efficiency), the thickness of the silicon “I” region becomes so large that the device's maximum bandwidth (also referred to as “data rate”) is too low for many current and future telecom and data center applications.
To avoid the inherent problem that silicon PDs and APDs have with longer wavelengths and higher bandwidths, other materials are used. Germanium (Ge) APDs detect infrared out to a wavelength of 2000 nm, but has relatively high multiplication noise. InGaAs APDs can detect out to longer than 1600 nm, and have less multiplication noise than Ge, but still far greater multiplication noise than silicon APDs. InGaAs is known to be used as the absorption region of a heterostructure diode, most typically involving InP as a substrate and as a multiplication layer. This material system is compatible with an absorption window of roughly 900 to 1700 nm. However, both InGaAs PD and APD devices are relatively expensive and have relatively high multiplication noise when compared with silicon and are difficult to integrate with Si electronics as a single chip.
Information published by a major company in the business of photodetectors (See http://files.shareholder.com/downloads/FNSR/0x0x382377/0b3893ea-fb06-417d-ac71-84f2f9084b0d/Finisar_Investor_Presentation.pdf) indicates at page 10 that the current market for optical communication devices is over 7 billion U.S. dollars with a compounded annual growth rate of 12%. Photodiodes (PD) used for 850-950 nm wavelength employ GaAs material and for 1550-1650 nm wavelength photodiodes are InP material based, which is both expensive and difficult to integrate with Si based electronics. Therefore, there is a large market and a long-felt need that has not been met for the development of a better device. To date there are no Si material based photodiodes nor avalanche photodiodes (APD) for 850-950 nm and no Ge on Si material based photodiodes nor avalanche photodiodes for 1550-1650 nm that are top-surface or bottom-surface illuminated, with a data rate of at least 25 Gb/s, and are monolithically integrated with CMOS/BiCMOS silicon electronics on a single chip that are commercially available to the knowledge of the inventors herein. However, there has been no lack of trying to develop a better device for this large market. For example, there have been proposals for resonant photodiodes fabricated in Si material (see Resonant-Cavity-Enhanced High-Speed Si Photodiode Grown by Epitaxial Lateral Overgrowth, Schaub et al., IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 11, NO. 12, December 1999), but they have not reached the known commercial market. Other forms of high speed photodiodes in a waveguide configuration have been proposed, such as in 40 GHz Si/Ge uni-traveling carrier waveguide photodiode, Piels et al, DOI 10.1109/JLT.2014.2310780, Journal of Lightwave Technology (incorporated herein by reference); Monolithic germanium/silicon avalanche photodiodes with 340 GHz gain-bandwidth product, NATURE PHOTONICS | VOL 3 | January 2009 | www.nature.com/naturephotonics (incorporated herein by reference and referred to herein as “Kang et al. 2009”); High-speed Ge photodetector monolithically integrated with large cross-section silicon-on-insulator waveguide, Feng et al., Applied Physics Letters 95, 261105 (2009), doi: 10.1063/1.3279129 (incorporated herein by reference); where light is coupled edge-wise into an optical waveguide and where the absorption length can be 100 um or longer to compensate for the weak absorption coefficient of Ge at 1550 nm. In these previously proposed waveguide photodiode structures, light propagates along the length of the waveguide and the electric field is applied across the PIN waveguide such that the direction of light propagation and the direction of the electric field are predominantly perpendicular in this waveguide configuration. Since light in Si travels approximately 1000 times faster than the saturated velocity of electrons/holes, a waveguide PD can be 200 microns long for example and the “I” in the PIN can be 2 microns, for example, and achieve a bandwidth of over 10 Gb/s. Such edge coupling of light is costly in packaging as compared to surface illumination as described in this patent specification, where dimensions of the waveguide cross section are typically a few microns as compared to tens of microns for known surface illuminated photodiodes or avalanche photodiodes. Known waveguide PD/APD are often only single mode optical systems whereas surface illuminated PD/APD described in this patent specification can be used in both single and multimode optical systems. In addition, known waveguide photodiodes are difficult to test at wafer level, whereas surface illuminated photodiodes described in this patent specification can be easily tested at wafer level. Known waveguide photodiodes/avalanche photodiodes are used mostly in specialty photonic circuits and in many cases require careful temperature control, which can be costly and inefficient in a hostile data center environment. A top or bottom illuminated Si and Ge on Si or GeSi on Si PD/APD that can be integrated with Si is not known to be commercially available at data rates of 25 Gb/s or more at wavelengths of 850-950 nm, 1250-1350 nm and 1550-1650 nm. In contrast, photodiodes on Si based material, as described in this patent specification, can be monolithically integrated with integrated electronic circuits on a single Si chip, thereby significantly reducing the cost of packaging. In addition, the microstructured PD/APD at 850 nm, 1300 nm and 1550 nm nominal wavelengths described in this patent specification can be predominantly for short haul (short reach), medium haul (reach gap) and long haul (long reach), distances less than 300 meters, in certain cases less than 2000 meters, in certain cases less than 10000 meters and in certain cases greater than 10000 meters optical data transmission. The microstructured PD/APD direction of incident optical beam and the electric field in the “I” region of a PIN or NIP structure, are predominately collinear and/or almost collinear. This patent specification enables such a device and is expected to transform the current data centers to almost all optical data transmission between blades, within a blade, between racks and/or between data centers, that will vastly increase the data transmission bandwidth capabilities and significantly reduce electrical power usage.
The subject matter claimed herein is not limited to embodiments that solve any specific disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one exemplary technology area where some embodiments described herein may be practiced.
Each published document referenced in this patent specification is hereby incorporated by reference.